Analyzer for comparative measurements of bulk conductivity

Abstract

Circuit for measuring the relative change in conductivity between two cells which include a test biorigion and a control biorigion as the test cell biorigion conductivity changes as a result of analyte modecules binding to receptor sites in the test cell biorigion. The invention includes circuitry for driving a current through each of the cells and for measuring the voltage drops across the test and control cells as a function of time. A novel two-stage ac analog-to-digital convertion circuit in which the initial conductivities are measured via a successive approximation process, while the relative conductivities are monitored during a test interval via a tracking technique is included. The circuitry is controlled by a digital controller which can vary the parameters of the measurement processes so as to make the circuit adaptable to the measurement of a wide variety of substances using different cell configurations and/or electrolytes.

What is claimed is: 
     
       1. Apparatus for measuring the relative impedances of a test cell and a control cell, including: signal generator means for causing AC current signals of a selected frequency to flow through the test cell and through the control cell so that the phase and magnitude of the current flowing through the test cell is substantially identical to the phase and magnitude of the current flowing through the control cell;   first amplifier means, responsive to the voltage drop across the test cell and including a first transformer having an output winding, for providing an output signal across the output winding proportional to the voltage drop across the test cell;   multiplier means reponsive to a first set of digital signals applied thereto for providing an output signal equal to a selected fractional part of the first transformer output signal;   second amplifier means, responsive to the voltage drop across the control cell and including a second transformer having a plurality of output windings, for providing output signals across each of the plurality windings proportional to the voltage drop across the control cell, the plurality of output windings having differing numbers of turns so that a signal proportional to the voltage drop across the test cell may be scaled over a range by connecting in series selected ones of the plurality of windings;   a plurality of switches responsive to a second set of digital signals applied thereto for selectively connecting in series individuals ones of the second transformer output windings to provide a combined output signal;   switching means for connecting the first transformer output signal in series with the combined output signal with opposite polarity;   means for measuring the the first transformer output signal to produce a first measurement signal;   means for measuring the signal across one or more output windings of the second transformer to produce a second measurement signal:   means responsive to the first and second measurement signals for applying signals to the plurality of switches so that selected ones of the plurality of output windings are connected in series so that the combined output signal approximates the first transformer output signal and for activating the switching means thereafter;   means for measuring the voltage resulting from the series-connected first and combined output signals and for providing a first set of digital signals to the multiplyer means so that its output signal approximates said resulting voltage; and   means for measuring the difference between the multiplier means output signal and the series-connected first and combined output signals and for providing a signal representative thereof.